If still running the factory-equipped engine, you can tell by the 8th letter of the VIN. 'E' = 305, '7' = 350. Also, on the SPID label on the trunk will be 'L03' if it's a 305 or 'L05' if 350. If the engine has been swapped, you'll have to check the bellhousing, which is difficult to do with the engine in the car, but can be done with an inspection mirror.

On Caprice sedans, only 9C1 police cars or cars with the LTZ package had 350's available from the factory.